The sun beat down on the shallow, sea-fed fields where Kim Seong-baek was forced to work without pay, day after 18-hour day mining the big salt crystals that blossomed in the mud around him. Half-blind and in rags, Kim grabbed another slave, and the two disabled men headed for the coast.

Far from the glittering steel-and-glass capital of Seoul, they were now hunted men on this remote island where the enslavement of disabled salt farm workers is an open secret.

"It was a living hell," Kim said in a recent series of interviews with The Associated Press whose details are corroborated by court records and by lawyers, police and government officials.

Lost, they wandered past asphalt-black salt fields sparkling with a patina of thin white crust. They could feel the islanders inspecting them. Everyone knew who belonged and who didn't.

Near a grocery, the store owner's son rounded them up and called their boss, who beat Kim with a rake and sent him back to the salt fields.

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Slavery thrives on rural islands off South Korea's rugged southwest coast, nurtured by a long history of exploitation and the demands of trying to squeeze a living from the sea.

Two-thirds of South Korea's sea salt is produced at more than 850 salt farms on dozens of islands in Sinan County, including Sinui island, where half the 2,200 residents work in the industry. Workers spend grueling days managing a complex network of waterways, hoses and storage areas.

Five times during the last decade, revelations of slavery involving the disabled have emerged. Kim's case prompted a nationwide government probe of thousands of farms and disabled facilities that found more than 100 workers who'd received no, or scant, pay.

Yet little has changed on the islands, according to a months-long investigation by the AP based on court and police documents and dozens of interviews with freed slaves, salt farmers, villagers and officials.

Although 50 island farm owners and regional job brokers were indicted, national police say no local police or officials will face punishment, despite multiple interviews showing some knew about the slaves and even stopped escape attempts.

Soon after the national investigation, activists and police found another 63 unpaid or underpaid workers on the islands, three-quarters of whom were mentally disabled.

Kim's former boss, Hong Jeong-gi, didn't respond to multiple requests for comment through his lawyer. He's set to appeal a 3 1/2 year prison sentence next week.

Other farmers often describe themselves as providing oases for the disabled and homeless.

"These are people who are neglected and mistreated," Hong Chi-guk, a 64-year-old salt farmer in Sinui, told the AP. "What alternative does our society have for them?"

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The night of July 4, 2012, Kim, who'd been homeless for a decade, was sleeping in a Seoul train station when a stranger offered him a place to stay and a job in the morning.

Hours later, he stood on a Sinui island salt farm. Hong had paid an illegal job agent the equivalent of about $700 for his new worker, according to court records.

The beatings began the first day on the farm for Kim, who's visually disabled and described in court documents as having the social awareness of a 12-year-old.

"Each time I tried to ask him something, his punch came first," Kim told the AP.

Only a week after his first escape was thwarted, Kim began to plan another.

He and the other slave, Chae Min-sik, again tried to find their way to a port. But the grocery owner's son, identified by officials only as Yoon, rounded them up again and called Hong.

After another beating, it was back to work.

Hong, Kim discovered, was an influential man, a former village head. Despite his fear, Kim ran again at the end of the month. Again, Yoon captured them.

Furious, the owner said that if Kim ran again, he'd get a knife in the stomach. Hong beat Kim so badly he broke Kim's glasses. He worked Kim so hard the slave was too tired to think about escape.

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The number of people enslaved is difficult to determine because of the transient work, the remoteness of the farms and the closeness -- and often hostility -- of the island communities. Social workers believe many slaves have yet to be found, and that investigations have so far been inadequate.

"If the recent investigation was done properly, then pretty much everyone on the island should've been taken to the police station and charged," said Kim Kang-won, an activist who participated in the recent investigation on Sinui. "The whole village knew about it."

Provincial police have vowed to inspect farms and interview workers regularly, but people familiar with the island confirm that slavery is rampant.

"The police chief would tell me that I'd eventually come to understand that this was how things on the island worked," said Cho Yong-su, a doctor who worked at the Sinui Island public health centre from 2006 to 2007.

Han Bong-cheol, a pastor in Mokpo who lived on Sinui Island for 19 years until June, sympathized with farmers forced to deal with disabled, incompetent workers. "They spend their leisure time eating snacks, drinking alcohol and smoking cigarettes. They are taken once or twice a year to Mokpo so they can buy sex. It's a painful reality, but it's a pain the island has long shared as a community."

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After a year and a half as a slave, Kim made one last bid for freedom.

He managed to mail a letter to his mother in Seoul. Kim's mother brought the letter, which gave directions to the farm, to Seo Je-gong, then a police captain.

Because Kim's letter noted collaboration between local police and salt farm owners, Seo and another Seoul officer went to the island posing as tourists who'd come to fish and buy salt. They visited Hong's home while he was away and found the slaves sitting on a mattress in a room without heat or hot water. Kim, Seo said, looked like a homeless person.

Kim was frightened and baffled, then relieved. "I am going to live," he said.

Chae initially refused to leave Sinui but was freed later after Seo found a 2008 missing person's report for Chae. He now lives in a Seoul shelter.

Yoon, who repeatedly captured Kim and Chae, was fined $7,500.

Kim, who lives in Seoul and occasionally works construction jobs, settled with Hong for about $35,000 in unpaid wages. He has nightmares and receives treatment for his injuries.

He also gets flustered when he talks about salt, disgusted when he sees it. "Just thinking about it makes me grind my teeth."